In the prior art, several methods of monitoring a bedridden patient in a hospital or a recuperation home for his vital signs have been used. Usually, these methods are implemented with leads or without leads. In the methods with leads, the patient has typically monitoring transducers attached to his body; these transducers in turn are electrically connected directly to pieces of equipment where transducer signals are read and interpreted. Because of the physical electrical connections between patient and equipment, the patient's movements are greatly impeded; the electrical connections tend to be entangled easily. Furthermore, there is ever present the danger of a malfunctioning piece of equipment causing an electrical shock to the directly connected patient. Therefore, the trend in patient monitoring has been toward leadless monitoring.
The leadless monitoring of patient in the prior art typically comprises a transmitter attached to the patient for transmission of signals showing the patient's vital signs and a receiver for collection and processing of these signals. Since the receiver is usually in close proximity, the transmission can be typically by very low frequency signals using a magnetic antenna. The advantages of low frequency transmissions have been adequately described in the prior art, e.g., by Andrzej B. Przedpelski, "`Near Field` Communication," r.f. design, March/April, 1980. But magnetically coupled antennas have one severe problem when applied to leadless monitoring: a magnetic antenna is not omnidirectional. Thus, there is an orientation of the transmitter with respect to the receiver which will not allow communication. In other words, there is an orientation creating a null between transmitter and receiver, and in patient monitoring where constant monitoring of the patient's vital signs is crucial, a null in communication can be fatal. With this method, the patient's movement, though not as much as with monitoring with leads, nevertheless is restricted again. And because of this disadvantage, low frequency leadless monitoring has not, if at all, been widely applied.
The method in accordance with the present invention overcomes the problem of null orientation with magnetic antenna coupling in leadless monitoring of patients and allows a patient greater freedom of movement than in the past. In accordance with the method, three equivalent orthogonally oriented magnetic antennas are obtained from selected combinations of output signals from four wire loop magnetic antennas in a plane. The combinations of signals received with these loop antennas are used for compound, or diversity, reception. Thus, no transmitting antenna orientation of the patient being monitored can cause a complete null in the receiver, since there is an equivalent antenna to receive the transmitted signals in three equivalent receiving axes and, being orthogonal to each other, there cannot be a simultaneous null in all three axes. Furthermore, the method in accordance with the invention allows all three receiving axes in one antenna to be located in one plane. This is especially desirable in the case of leadless hospital monitoring; the planar antenna can be easily laid down flat on a hospital bed under the mattress of the patient. The receiving antenna is then unobtrusive yet in close proximity to the patient to take advantage of magnetic coupling with very low frequency signals.